Copper proteins monooxygenase activity

As the focus of this review is on copper-dioxygen chemistry, we shall briefly summarize major aspects of the active site chemistry of those proteins involved in 02 processing. The active site structure and chemistry of hemocyanin (He, 02 carrier) and tyrosinase (Tyr, monooxygenase) will be emphasized, since the chemical studies described herein are most relevant to their function. The major classes of these proteins and their origins, primary functions, and leading references are provided in Table 1. Other classes of copper proteins not included here are blue electron carriers [13], copper-thiolate proteins (metallothioneines) [17], and NO reductases (e.g., nitrite [NIR] [18] or nitrous oxide [19]). 

Tyrosinase (see Copper Proteins with Dinuclear Active Sites), a copper metalloenzyme with a very broad phylogenetic distribution, is responsible for the browning of fruits and mushrooms.Tyrosinase is a bifimctional phenol oxidase that is able to both hydroxylate monophenols like tyrosine (monooxygenase reaction, (equations)) and snbseqnently oxidize the diphenol product to the corresponding quinone (oxidase reaction, (equation 6)) at a single Type 3 binuclear copper active site. 

The application of direct electrochemistry of small redox proteins is not restricted to cytochrome c. For example, the hydroxylation of aromatic compounds was possible by promoted electron transfer from p-cresol methylhydroxylase (a monooxygenase from Pseudomonas putida) to a modified gold electrode [87] via the blue copper protein azurin. All these results prove that well-oriented non-covalent binding of redox proteins on appropriate electrode surfaces increases the probability of fast electron transfer, a prerequisite for unmediated biosensors. Although direct electron-transfer reactions based on small redox proteins and modified electrode surfaces are not extensively used in amperometric biosensors, the understanding of possible electron-transfer mechanisms is important for systems with proteins bearing catalytic activity. 

Tyrosinase is a monooxygenase which catalyzes the incorporation of one oxygen atom from dioxygen into phenols and further oxidizes the catechols formed to o-quinones (oxidase action). A comparison of spectral (EPR, electronic absorption, CD, and resonance Raman) properties of oxy-tyrosinase and its derivatives with those of oxy-Hc establishes a close similarity of the active site structures in these proteins (26-29). Thus, it seems likely that there is a close relationship between the binding of dioxygen and the ability to "activate" it for reaction and incoiporation into organic substrates. Other important copper monooxygenases which are however of lesser relevance to the model studies discussed below include dopamine p-hydroxylase (16,30) and a recently described copper-dependent phenylalanine hydroxylase (31). 

Polyphenol oxidase (PPO) (EC 1.14.18.1 monophenol monooxygenase [tyrosinase] or EC 1.10.3.2 0-diphenol 02-oxidoreductase) is one of the more important enzymes involved in the formation of black tea polyphenols. The enzyme is a metallo-protein thought to contain a binudear copper active site. The substance PPO is an oligomeric particulate protein thought to be bound to the plant membranes. The bound form of the enzyme is latent and activation is likely to be dependent upon solubilization of the protein (35). PPO is distributed throughout the plant (35) and is localized within in the mitochondria (36), the cholorplasts (37), and the peroxisomes (38). Using antibody techniques, polyphenol oxidase activity has also been localized in the epidermis palisade cells (39). Reviews on the subject of PPO are available (40—42). 

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